molecular distillation and packed columm distillation

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Submitted to:- Mr. M. Alam HOD (Chemical Department) Submitted by:- Kuldeep Patidar B.tech 3 rd year 5 th sem. 12MUBCHE003 Chemical Engineering

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Submitted to:-

Mr. M. Alam

HOD (Chemical Department)

Submitted by:-

Kuldeep Patidar

B.tech 3rd year 5th sem.

12MUBCHE003

Chemical Engineering

Molecular Distillation

Theory of molecular distillation:-

The mean free path of a molecule is defined as the average distance

through which a molecule can move without coming into collision with

another. For material that are regarded as non volatile under ordinary

conditions of temperature and pressure are generally removed by this by

increasing the mean free path.

Characteristics of the molecular distillation process:-

Very high vacuum

Evaporating surface must be close to the condensing surface

The liquid area is large to avoid boiling and evolution of the vapors is

from surface only.

Molecular Distillation (cont.)

• Application of molecular distillation:-

Purification of oils

Separating of vitamins

Packed distillation columns

Packed columns are used for distillation, gas absorption and liquid-liquid extraction.

The gas-liquid contact in a packed column is continuous, not stage-wise, as in a plate column.The liquid flows down in the column over a packing surface and the vapor (or the gas) movescounter-currently, up the column. The performance of a packed column is very dependent onthe maintenance of good liquid and gas distribution through the packed bed, and this is animportant consideration in packed column design.

Advantages and disadvantages for the plate

and the packed columns

1. Plate columns can be designed to handle a wider range of liquid and

gas flow ratesthan packed columns.

2. Packed columns are not suitable for very low liquid flow rates.

3. The efficiency of a plate can be predicted more accurately than the

equivalent terms ofpackings (HETP or HTU).

4. For corrosive liquids a packed column will usually be cheaper than the

equivalentplate column.

5.The liquid hold-up is lower in a packed column than in a plate

column. This can beimportant when the hold-up of toxic or flammable

liquids must be kept as small aspossible for safety reasons.

6. Packed columns are more suitable for handling foaming systems.

7. The pressure drop can be lower in a packed column than the

equivalent plate column.

The design procedure of a packed

column consists of the following steps:-

1. Select the type and size of packing.

2. Determine the column height required for the

specified separation.

3. Determine the column diameter (capacity), to

handle the liquid and vapor flow rates.

4. Select and design the column internal features:

packing support, liquid distributor and

redistributors.

1. Determination of the column height

1.1. The theoretical stage method

If at a point of the distillation column the concentration of the liquid flow is x, and theconcentration of the counter-current vapor flow at the same point is y, then there is a heightabove this point where the concentration of the vapor flow (y*) is in equilibrium with x. This section is the Height Equivalent a Theoretical Plate (HETP).

The value of ma can be determined by the following method:

1.2. The transfer unit method

The transfer unit is a part of column height where the change in vapor concentration equals tothe average driving force. The driving force is the difference between the equilibrium (y*) andthe actual (y) vapor concentration.

The column height can be computed by Eq. 4.

H = HTU ⋅ NTU _________________________(4)

where HTU Height of a Transfer Unit [m]

NTU Number of Transfer Units [-]